Crossword Genius

Birds in uniform? (5)

Ross

I believe the answer is:

wrens

I'm a little stuck... Click here to teach me more about this clue!

'birds' is the definition.
(wren is a kind of bird)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Another definition for wrens that I've seen is " Small brown birds".)

I've seen this clue in the Evening Standard.
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